Method and apparatus for increasing the number of orthogonal signals using block spreading

ABSTRACT

Embodiments of the invention apply block spreading to transmitted signals to increase the number orthogonally multiplexed signals. The principle of the disclosed invention can be applied to reference signals, acknowledgement signals, and channel quality indication signals. In any given time interval, the set of transmitted signals is defined by two sequences: the baseline sequence, and the block spreading sequence. Different transmitters using the same baseline sequence can be identified by using different block spreading sequences.

CLAIM OF PRIORITY UNDER 35 U.S.C. §119

The present application for Patent claims priority to U.S. ProvisionalApplication No. 60/762,071 entitled “Increasing the Number of OrthogonalPilot Channels” filed Jan. 25, 2006. All applications assigned to theassignee hereof and hereby expressly incorporated by reference herein.

BACKGROUND

Embodiments of the invention are directed, in general, to wirelesscommunication systems and can be applied to generation and multiplexingof signals in multi-user wireless communications systems based onsingle-carrier frequency division multiple access (SC-FDMA) andorthogonal frequency division multiple access (OFDMA).

FIG. 1 shows a block diagram of a transmitter 110 and a receiver 150 ina wireless communication system 100. For simplicity, transmitter 110 andreceiver 150 are each equipped with a single antenna but in practicethey may have two or more antennas. For the downlink (or forward link),transmitter 110 may be part of a base station (also referred to as NodeB), and receiver 150 may be part of a terminal (also referred to as userequipment—UE). For the uplink (or reverse link), transmitter 110 may bepart of a UE, and receiver 150 may be part of a Node B. A Node B isgenerally a fixed station and may also be called a base transceiversystem (BTS), an access point, or some other terminology. A UE, alsocommonly referred to as terminal or mobile station, may be fixed ormobile and may be a wireless device, a cellular phone, a personaldigital assistant (PDA), a wireless modem card, and so on.

At transmitter 110, a reference signal (also referred to as pilotsignal) processor 112 generates reference signal symbols (or pilotsymbols). A transmitter (TX) data processor 114 processes (e.g.,encodes, interleaves, and symbol maps) traffic data and generates datasymbols. As used herein, a data symbol is a modulation symbol for data,a reference signal symbol is a modulation symbol for reference signal,and the term “modulation symbol” refers to a real valued or complexvalued quantity which is transmitted across the wireless link. Amodulator 120 receives and multiplexes the data and reference symbols,performs modulation on the multiplexed data and reference symbols, andgenerates transmission symbols. A transmitter unit (TMTR) 132 processes(e.g., converts to analog, amplifies, filters, and frequencyup-converts) the transmission symbols and generates a radio frequency(RF) modulated signal, which is transmitted via an antenna 134.

At receiver 150, an antenna 152 receives the RF modulated signal fromtransmitter 110 and provides a received signal to a receiver unit (RCVR)154. Receiver unit 154 conditions (e.g., filters, amplifies, frequencydown-converts, and digitizes) the received signal and provides inputsamples. A demodulator 160 performs demodulation on the input samples toobtain received symbols. Demodulator 160 provides received referencesignal symbols to a channel processor 170 and provides received datasymbols to a data detector 172. Channel processor 170 derives channelestimates for the wireless channel between transmitter 110 and receiver150 and estimates of noise and estimation errors based on the receivedreference signal. Data detector 172 performs detection (e.g.,equalization or matched filtering) on the received data symbols with thechannel estimates and provides data symbol estimates, which areestimates of the data symbols sent by transmitter 110. A receiver (RX)data processor 180 processes (e.g., symbol demaps, deinterleaves, anddecodes) the data symbol estimates and provides decoded data. Ingeneral, the processing at receiver 150 is complementary to theprocessing at transmitter 110.

Controllers/processors 140 and 190 direct the operation of variousprocessing units at transmitter 110 and receiver 150, respectively. Forexample, controller processor 190 may provide demodulator 160 with areplica of the reference signal used by reference signal processor 112in order for demodulator to perform possible correlation of the twosignals. Memories 142 and 192 store program codes and data fortransmitter 110 and receiver 150, respectively.

The disclosed invention is applicable, but not restricted to, frequencydivision multiplexed (FDM) reference signal transmission forsimultaneous transmission from multiple UEs. This includes, but is notrestricted to, OFDMA, OFDM, FDMA, DFT-spread OFDM, DFT-spread OFDMA,single-carrier OFDMA (SC-OFDMA), and single-carrier OFDM (SC-OFDM) pilottransmission. The enumerated versions of FDM transmission strategies arenot mutually exclusive, since, for example, single-carrier FDMA(SC-FDMA) may be realized using the DFT-spread OFDM technique. Inaddition, embodiments of the invention also apply to generalsingle-carrier systems.

FIG. 2 is an example of a block diagram showing an OFDM(A) transmitterof the reference signal (RS). It comprises of the RS sequence generator201 and the Modulate block 202, which generate a reference signal block203. Samples 203 are transmitted over the air. Modulate block furtherconsists of a Tone Map 202A, insertion of zeros or other signals 202B,and the IFFT in 202C. Tone Map 202A can be arbitrary. Elements ofapparatus may be implemented as components in a programmable processoror Digital Signal Processor (DSP).

FIG. 3 is an example of a block diagram showing a DFT-spread OFDM(A)(bracketed letter “A” means that the statement holds for both DFT-spreadOFDM and DFT-spread OFDMA) reference signal (RS) transmitter. Itcomprises of the RS sequence generator 301 and the Modulate block 302,which generate a reference signal block 303. Samples 303 are transmittedover the air. Modulate block further consists of: DFT 302D, Tone Map302A, insertion of zeros or other signals 302B, and the IFFT in 302C.Tone Map 302A can be arbitrary. Elements of apparatus may be implementedas components in a programmable processor or Digital Signal Processor(DSP).

Embodiments of the invention will be described using a family ofmathematically well studied sequences, known as CAZAC sequences, astransmitted reference signals for several purposes including coherentdemodulation of the data signal and possible channel quality estimation.CAZAC sequences are defined as all complex-valued sequences with thefollowing two properties: 1) constant amplitude (CA), implying thatmagnitudes of all sequence elements are mutually equal and 2) zerocyclic autocorrelation (ZAC). Well-known examples-of CAZAC sequencesinclude (but are not limited to) Chu and Frank-Zadoff sequences (orZadoff-Chu sequences), and generalized chirp like (GCL) sequences.Nevertheless, the use of CAZAC reference signals is not mandatory forthis invention. There is a need to define reference signal (RS)generation and transmission such that multiple reference signals can besimultaneously orthogonally multiplexed. Such generation should allowefficient use of the RS resources, which will in turn maximize thenumber of simultaneously multiplexed RS transmitters. Although theexemplary embodiment considers for brevity RS generation andmultiplexing, exactly the same principles can be used to orthogonallymultiplex other signals and increase their number, includingacknowledgement signals (ACK/NAK) and channel quality indication (CQI)signals.

SUMMARY

In light of the foregoing background, embodiments of the inventionprovide an apparatus, method and system for generating, multiplexing andallocating reference signals to multiple transmitters. The proposedmethod can produce orthogonally multiplexed signals among the multipletransmitters thereby avoiding corresponding mutual interference.

The exemplary embodiment of the invention considers the generation ofreference signals (RS) using constant amplitude zero cyclicauto-correlation (CAZAC) sequences, and block spreading, formultiplexing RS from multiple transmitters. RS can be used for thepurposes of coherent data (and/or control) signal demodulation, channelquality estimation, and other functionalities discussed herein. The sameexactly principle of block spreading of CAZAC sequences can be extendedto the multiplexing of other signals such as acknowledgement signals(ACK/NAK) related to a packet transmission or channel quality indication(CQI) signals. The proposed generation and multiplexing method of RS canbe applied to all frequency division multiplex (FDM) systems which areused by multiple UEs, with or without multiple transmit antennas. Thisincludes, but is not restricted to OFDMA, OFDM, FDMA, DFT-spread OFDM,DFT-spread OFDMA, single-carrier OFDMA (SC-OFDMA), and single-carrierOFDM (SC-OFDM) RS transmission.

System and method of embodiments of the present invention solve problemsidentified in prior techniques and provide additional advantages.

BRIEF DESCRIPTION OF THE DRAWINGS

Having thus described the invention in general terms, reference will nowbe made to the accompanying drawings, which are not necessarily drawn toscale (for example, the number of sub-carriers in FIG. 2 through FIG. 7may be substantially larger than illustrated, such as tens, hundreds orthousands of sub-carriers), and wherein:

FIG. 1 is a diagram illustrative of an exemplary wireless communicationsystem;

FIG. 2 is a diagram illustrative of an exemplary OFDM(A) referencesignal transmitter;

FIG. 3 is diagram illustrative of an exemplary DFT-spread OFDM(A)reference signal transmitter;

FIG. 4 is a block diagram showing an apparatus for reference signalgeneration in accordance with a first embodiment of the invention;

FIG. 5 is a block diagram showing an apparatus for reference signalgeneration in accordance with a second embodiment of the invention;

FIG. 6 is a block diagram showing an apparatus for reference signalgeneration in accordance with a third embodiment of the invention;

FIG. 7 is a block diagram showing an apparatus for reference signalgeneration in accordance with a fourth embodiment of the invention;

FIG. 8 is a block diagram showing an apparatus for reference signalreception in accordance with the embodiment of the invention describedin FIG. 6.

DETAILED DESCRIPTION

The invention now will be described more fully hereinafter withreference to the accompanying drawings. This invention may, however, beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein. Rather, these embodiments areprovided so that this disclosure will be thorough and complete, and willfully convey the scope of the invention to those skilled in the art.

This invention will be described using CAZAC sequences as baselinesequences for RS generation. CAZAC sequences are well-described in theliterature and can be found in several publications. For example, theyare studied in the article by A. Milewski, “Periodic sequences withoptimal properties for channel estimation and fast start-upequalization”, IBM Journal of Research & Development, vol. 27, No. 5,September 83, pages 426-431. CAZAC sequences include a category ofsequences that are polyphase sequences. See for example: L. H.Zetterberg “A class of codes for polyphases signals on a band-limitedgaussian channel”, IEEE Trans. on Info. Theory, IT-11, pp 385, 1965;also see, A. J. Viterbi “On a class of polyphases codes for the coherentgaussian channel”, IEEE Int. Cony. Record, Part 7, pp 209, 1965. D. C.Chu “Polyphase Codes with Good Periodic Correlation Properties.” IEEETrans. Info. Theory IT-18, pp. 531-532 (July 1972). CAZAC sequences alsoinclude the so-called generalized chirp like (GCL) sequences, as shownin the reference B. M. Popovic, “Generalized Chirp-like PolyphaseSequences with Optimal Correlation Properties,” IEEE Trans. Info.Theory, vol. 38, pp. 1406-1409, July 1992. See also, U.S. Pat. No.3,008,125 by Zadoff et al.

As a specific example of a CAZAC sequences, we cite the formula for theZadoff-Chu (ZC) family of CAZAC sequences given in page 53 from K. Fazeland S. Keiser, “Multi Carrier and Spread Spectrum Systems,” John Willeyand Sons, 2003. Let L be any positive integer, and let k be any numberwhich is relatively prime with L. Also, let q be any integer. Then,according to the provided reference, the n-th entry of the k-thZadoff-Chu CAZAC sequence is given as follows

${c_{k}(n)} = {{\exp \left\lbrack {j\; 2\pi \; k\; \frac{{n{\left( {n + 1} \right)/2}} + {qn}}{L}} \right\rbrack}\mspace{14mu} {for}\mspace{14mu} L\mspace{14mu} {odd}}$${c_{k}(n)} = {{\exp \left\lbrack {j\; 2\pi \; k\; \frac{{n^{2}/2} + {qn}}{L}} \right\rbrack}\mspace{14mu} {for}\mspace{14mu} L\mspace{14mu} {even}}$

The set of Zadoff-Chu CAZAC sequences has following desirable properties(regardless of the value of q)

-   -   Constant magnitude (or constant amplitude). This property is        valid for generic CAZAC sequences, and is not specific to the        Zadoff-Chu family.    -   Zero circular auto-correlation. This property is valid for        generic CAZAC sequences, and is not specific to the Zadoff-Chu        family.    -   Flat frequency domain response. This means that the magnitudes        of each DFT entry of a CAZAC sequence are all equal. It can be        shown that this property is mathematically equivalent to zero        circular auto-correlation property. Thus, this property is valid        for generic CAZAC sequences, and is not specific to the        Zadoff-Chu family.    -   Circular cross-correlation between two sequences is low and with        constant magnitude which is independent of the sequence offset.        This property is specific to the Zadoff-Chu CAZAC sequences        (with the integer q being fixed) and prime L.

From a base family of CAZAC sequences, additional sequences can begenerated using any of the following operations on each individualsequence: multiplication by a complex constant, DFT, IDFT, FFT, IFFT,cyclic shift, and block-repetition (and, under certain conditions,sequence truncation). With block-repetition, the zero cyclicauto-correlation property holds only up to a certain delay. Thus, withblock-repetition, the cyclic auto-correlation is zero in the vicinity ofthe peak (this property is also referred to as pseudo-CAZAC).Nevertheless, the disclosed invention does not preclude the use of suchpseudo-CAZAC sequences. Furthermore, the disclosed invention does notpreclude the use of sequences which are generated from other base CAZACsequences, using any of the described operations (multiplication by acomplex constant, DFT, IDFT, FFT, IFFT, cyclic shift, block-repetition,truncation), or a combination thereof.

The use of CAZAC sequences in this invention is only exemplary. Othersequences with desirable auto and cross-correlation properties can beused as well, in conjunction with the disclosed invention, as follows.

FIG. 4 is a block-diagram showing an apparatus in accordance with anembodiment of the invention. Apparatus 400 comprises from: baseline RSgenerator 401, RS Modulate block 402, complex multiplier block 406, andthe block-spreading sequence generator 403. In turn, the block spreadingsequence generator 403 comprises of sub-blocks: 403.0 which generatesfirst entry of the block-spreading sequence, 403.1 which generatessecond entry of the block-spreading sequence, etc, until the 403.(T−1)which generates the last entry of the block-spreading sequence. Elementsof apparatus may be implemented as components in a programmableprocessor or Digital Signal Processor (DSP).

In one embodiment of the invention, each of RS blocks (404.0, 404.1,etc, 404.(T−1)), from a time interval 405, is generated using blockspreading, as follows. To generate each RS block, the baseline RSgenerator 401 generates an RS sequence. Generated baseline RS sequenceis then passed to the “Modulate” block 402. The “Modulate” block can beany one of the modulators shown in the prior art (for example, FIG. 2 orFIG. 3), but this is not mandatory. Subsequently, the entire modulatedRS sequence is block-multiplied with a block-spreading entry (403.0 or403.1, etc, or 403.(T−1)) of the block-spreading sequence 403, using thecomplex multiplier block 406. The block spreading sequence 403 may haveelements exclusively comprising of +1 and −1 in which case themultiplication block 406 may simply be replaced by sign flips or noflips. To generate RS block 404.0, the block-spreading entry 403.0 isused, to generate RS block 404.1, the block-spreading entry 403.1 isused, etc, and to generate RS block 404.(T−1), the block-spreading entry403.(T−1) is used. Obviously, at times, a number of computations can besaved by performing 401 and 402 only once per time interval 405. Dataand/or control transmission can occur in between RS blocks.

FIG. 5 is another block-diagram showing an apparatus in accordance withan embodiment of the system. In contrast to FIG. 4, the apparatus fromFIG. 5 performs block-spreading prior to the modulation. Apparatus 500comprises from: baseline RS sequence generator 501, complex multiplier506, block-spreading sequence generator 503, which generatesblock-spreading entries (503.0, 503.1, etc, 503.(T−1)) of theblock-spreading sequence, and finally, the series of modulator blocks502.0, 502.1, etc, 502.(T−1). Each of the modulator blocks can be one ofthe modulators shown in the prior art (for example, FIG. 2 or FIG. 3),but this is not mandatory. Each of the modulator blocks can operate on adifferent set of data.

In another embodiment of the invention, each of RS blocks 503.0, 503.1,etc, 503.(T−1) from the time interval 504 is generated usingblock-spreading, as follows. First, the baseline RS generator block 501generates the baseline RS sequence. To generate the RS block “t” (wheret take on values 0,1, . . . , T−1), the entire baseline RS sequence ismultiplied by the block-spreading entry 503.t, using the multiplier 506,and then modulated using the “Modulate” block 502.t. At times, a numberof computations can be saved by performing 501 only once pertransmission time interval. Data and/or control transmission can occurin between RS blocks. In a number of different scenarios, embodimentfrom FIG. 5 can be made equivalent to the embodiment from FIG. 4.

To separate different transmitters, prior art methods consider using adifferent baseline RS sequence for different transmitters. The disclosedinvention considers that different transmitters can also be separatedwhen they are using an identical (or correlated) baseline RS sequence,but with different block-spreading sequences. These differenttransmitters can be either: a) different mobiles, b) differentbase-stations, c) different antennas from the same mobile, d) differentantennas from the same base-station, or e) any combination thereof.Thus, disclosed invention allows a system designer to increase the totalnumber of different RS signals through orthogonal multiplexing by afactor which is the total number of used block-spreading sequences. Theblock-spreading sequence is denoted as s_(m)(t), which further denotesthe t-th entry of the m-th block spreading sequence. Multiple choicesfor block spreading sequences exist and any set of sequences with goodcorrelation properties can be used. For example, the conventional Walshsequences can provide such a set of block spreading sequences. It isalso possible to use cyclic shifts of a root CAZAC sequence, to generatedifferent block-spreading sequences.

To illustrate how block-spreading can be used to separate differenttransmitters, we now turn to FIG. 6, illustrating an exemplary timeinterval 605 containing two distinct RS blocks, namely 604.0 and 604.1.Other blocks can carry data and control, and all blocks are preceded bycyclic-prefix transmission (CP), as common in OFDM-based systems. Twodifferent transmitters which use a common baseline RS sequence[c_(k)(0), c_(k)(1), . . . , c_(k)(L−1)], generated by 601, can beseparated using orthogonal block-spreading sequences. For onetransmitter, the block-spreading sequence generator 603.0 and 603.1 cangenerate entries s₀=[s₀(0), s₀(1)]=[+1, +1]. For another transmitter,the corresponding block-spreading sequence generator 603.0 and 603.1 cangenerate entries using s₁=[s₁(0), s₁(1)]=[+1, −1]. The previous blockspreading sequences are the well known Walsh sequences with length 2.The modulator block 602 can be one of the modulators shown in the priorart (for example, FIG. 2 or FIG. 3), but this is not mandatory. Themultiplier block 606 in case of Walsh sequences can be a simple signoperator according to the corresponding sign of the Walsh sequenceelement. In this specific case, the multiplier block 606 only flips thesign bit for the second transmitter in the second RS block. Also, eachtransmitter can reduce computation by executing 601 and 602 only onceper time interval 605.

A number of different receiver structures can be applied to thedisclosed invention. For example, the receiver structure in. FIG. 8corresponds to a transmitter structure in FIG. 6 (additional receiverstructures corresponding to the remaining transmitter configurations arestraightforward to derive and are omitted for brevity). Receiver 800first performs block de-spreading for the received RS signal which iseventually used for channel estimation. Block de-spreading is performedon received RS blocks 804.0 and 804.1, using the multiplier 806 andadder 802. Here, the received blocks are first block-multiplied bycomplex conjugates of the corresponding block-spreading sequence (803.0or 803.1), and then block-added using 802. Further channel estimationoperations are performed by 801 to arrive at channel estimates 807. Onceagain, in case of Walsh block spreading sequences comprising of +1 and−1 values, complex conjugates and multiplication are not needed as thede-spreading operation is simply the appropriate sign applicationfollowed by addition over the Walsh sequence elements. Finally, notethat an alternate receiver structure can first perform channelestimation (RS demodulation etc) first, and then follow up byblock-de-spreading.

To further illustrate how block-spreading can separate differenttransmitters, we now turn to FIG. 7, further illustrating anotherexemplary structure containing four distinct RS blocks, namely 704.0,704.1, 704.2 and 704.3. Other blocks can carry data and control, and allblocks are preceded by cyclic-prefix transmission (CP), as common inOFDM-based systems. Two different transmitters which use a common (orjust correlated) baseline RS sequence [c_(k)(0), c_(k)(1), . . . ,c_(k)(L−1)], generated by 701, can be separated using orthogonalblock-spreading sequences. For one transmitter, the block-spreadingsequence generator 703.0, 703.1, 703.2, 703.3 can generate entriess₀=[s₀(0), s₀(1) s₀(2), s₀(3)]=[+1, +1, +1, +1]. For anothertransmitter, the block-spreading sequence generator 703.0, 703.1, 703.2,and 703.3 can generate entries s₁=[s₁(0), s₁(1) s₁(2), s₁(3)]=[+1, −1,+1, −1]. The previous block spreading sequences are the well known Walshsequences with length 4. The modulator block 702 can be one of themodulators shown in the prior art (for example, FIG. 2 or FIG. 3), butthis is not mandatory.

Thus, with the proposed block-spreading transmission of the RS, theentire RS transmission, for a particular mobile, is defined using twosequences: the baseline RS sequence [c_(k)(0), c_(k)(1), . . . ,c_(k)(L−1)], and the block-spreading sequence [s_(m)(0), s_(m)(1) . . .s_(m)(T−1)] which in the examples of FIG. 6 and FIG. 7 is a Walshsequence. Each of these two sequences has to be assigned to the mobile.This is done, explicitly or implicitly, by the base-station serving (orcontrolling) the reference mobile.

To maintain (near) orthogonality among the simultaneously multiplexedsignals through the exemplary block-spreading in FIG. 4 or FIG. 5 orFIG. 6 or FIG. 7, it is assumed that the channel does not changesubstantially in the time period between the transmissions of differentRS blocks. Validity of this assumption can be determined by theassigning Node B though estimation of the Doppler shift (or Dopplerspread) of the mobiles. Thus, the controlling node-B may multiplexslow-moving mobiles using block-spreading. Any additional fast movingmobile can then be multiplexed using a different baseline RS sequence(for example c₂(n)), or a different cyclic shift, or using a differentset of tones.

All herein described reference signal transmissions (or parts of them)may be pre-computed, stored in the memory of the UE device, and usedwhen necessary. Any such operation (pre-computing and storage) does notlimit the scope of the invention.

The exemplary embodiment of the invention assumes that the referencesignal is time division multiplexed (TDM) with the data and/or controlsignal (from a single UE), that is, transmission of the reference signaldoes not occur concurrently with the data and/or control signal. Thisassumption only serves to simplify the description of the invention, andis not mandatory to the invention. Nevertheless, when the referencesignal is TDM multiplexed with the data signal, the two can usedifferent modulation. For instance, data signal can use SC-OFDM(A),while the reference signal can use OFDMA modulation.

In case of multi-antenna transmission, multiple antennas of a singe UEcan be treated as different UEs (different transmitters), for thepurpose of allocating reference signals. All herein described designsextend in a straightforward manner to the case of multi-antennatransmission.

All herein described multi-user allocations can be trivially reduced andalso applied to the single-user scenario.

The principle of “block spreading” also applies to the multiplexing ofother signals, such as acknowledgement (ACK/NAK) and channel qualityindicator (CQI) signals from different UEs. In this case, different UEscan use different sequences [c_(k)(0), c_(k)(1), . . . , c_(k)(L−1)] or[s_(m)(0), s_(m)(1) . . . s_(m)(T−1)], that are modulated with aninformation symbol which is identical to described embodiments. Forexample, an ACK transmission may correspond to the transmission of thesame sequence as for the RS while a NAK transmission may correspond toits algebraic opposite. For CQI transmission, complex modulation symbolscan be used to scale the transmitted sequence. Thus, embodiments of theinvention can also be applied beyond the RS transmission.

Many other modifications and other embodiments of the invention willcome to mind to one skilled in the art to which this invention pertainshaving the benefit of the teachings presented in the foregoingdescriptions, the associated drawings, and claims. Therefore, it is tobe understood that the invention is not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

All herein described reference signal transmissions may (or may not) bepreceded by a “cyclic prefix,” which is a common practice in allfrequency division multiplex systems (FDM). These systems include, butare not restricted to, OFDM, OFDMA, FDMA, DFT-spread OFDM, DFT-spreadOFDMA, single-carrier OFDMA (SC-OFDMA), and single-carrier OFDM(SC-OFDM) pilot transmission. Transmission (or non-transmission) of the“cyclic prefix” doesn't affect the scope of the invention.

Definition of “time interval,” during which the above block-spreading isapplied, in the exemplary embodiments, can be understood as anypre-determined time unit. For example, it “time interval” can correspondto frame, sub-frame, transmission time interval, slot, or any other timeunit.

At times, RS block-spreading may simply be performed just to achieveinterference randomization. In this case, RS block-spreading can beimplemented using either “short” or “long” block-spreading sequences.

1-24. (canceled)
 25. A method of separating transmitters, comprising thesteps of: receiving a signal transmitted by a first transmitter using afirst baseline sequence employing a first block-spreading sequence;receiving a signal transmitted by a second transmitter using the firstbaseline sequence employing a second block-spreading sequence; and usingthe first and second block-spreading sequences to separate the first andsecond transmitters.
 26. The method of claim 25, wherein said firstsequence is based on a constant amplitude zero cyclic auto-correlation(CAZAC) sequence.
 27. The method of claim 26, wherein said CAZACsequence is a Zadoff-Chu sequence.
 28. The method of claim 25, whereinsaid signal is one of: a reference signal; an acknowledgement signal;and a channel quality indication signal.
 29. An apparatus, comprising:circuitry for receiving a signal transmitted by a first transmitterusing a first baseline sequence employing a first block-spreadingsequence; circuitry for receiving a signal transmitted by a secondtransmitter using the first baseline sequence employing a secondblock-spreading sequence; and circuitry for using the first and secondblock-spreading sequences to separate the first and second transmitters.30. The apparatus of claim 29, wherein the first baseline sequence isbased on a constant amplitude zero cyclic auto-correlation (CAZAC)sequence.
 31. The apparatus of claim 30, wherein said CAZAC sequence isa Zadoff-Chu sequence.
 32. The apparatus of claim 29, wherein saidsignal is one of: a reference signal; an acknowledgement signal; and achannel quality indication signal.